The present invention relates to a composition for controlling the amount of air pollution emitted from combustion gasses, and in particular to alkaline earth metal polysulfides for removing mercury from flue gasses. The invention also relates to the use of the compositions to reduce air pollution by removing mercury and other pollutants from combustion exhaust gasses, and the methods employed to accomplish such reduction of air pollution.
Combustion gasses from incinerators, power plants, and coal-fired furnaces typically contain oxides of sulfur (SOx), oxides of nitrogen (NOx), and volatile heavy metals such as mercury. On combustion, the mercury is volatilized and carried in the combustion exhaust gasses into the atmosphere.
Coal-burning electric power plants are the single biggest source of mercury emissions, accounting for 40 percent of the total mercury emitted from all man-made sources. Coal-fired burners account for another 10 percent. Coal fired power stations burning high sulfur bituminous and sub-bituminous coal combustion gases typically discharge combustion gases containing 10-20 μg/Nm3 total mercury and 1 to 3 μg/Nm3 elemental mercury (Hg0) from their electrostatic precipitation (ESP) systems. On entering a wet FGD system the ionized and oxidized portion of the total mercury (HgX) becomes largely dissolved in the scrubber SOx absorption media with approximately 5% of the inbound HgX passing through the system. Wet FGD absorption media are typically 25-30% w/w solids dispersions of calcium carbonate, magnesium carbonate, or a mixture thereof and their respective sulphites and sulphates.
Mercury poses a serious problem for human beings and the environment and as such protection from exposure to mercury pollution has been the subject of US legislation resulting in The Clean Air Mercury Rule of Mar. 15, 2005 and the EPA's Clean Air Interstate Rule (CAIR). Mercury, atomic symbol Hg, is a persistent, bioaccumulative toxic metal that is emitted in combustion gasses in three forms: Elemental mercury, Hg0, oxidized mercury, Hg2+ compounds, and particle-bound mercury. After mercury has precipitated from the air and deposited into bodies of water or onto the land, methylmercury is formed by microbial action in the top layers of sediment and soils. Once formed, methylmercury is taken up by aquatic organisms and bioaccumulates up the aquatic food web. Methylmercury is a well-established human neurotoxicant. Methylmercury that is ingested by humans is readily absorbed from the gastrointestinal tract and can cause effects in several organ systems.
The aim of these regulations is to significantly reduce emissions from coal-fired power plants, the largest remaining sources of mercury emissions in the US. When fully implemented, these rules will reduce utility emissions of mercury from 48 tons a year to 15 tons, a reduction of nearly 70 percent. Typical mercury concentrations in coal are 0.05 to 0.25 mg/Kg. The typical discharge concentrations of total mercury, largely in its elemental form are in the range 2 to 6 μg/Nm3, (where Nm3 is Non-IUPAC nomenclature. N or “normal” refers to gas volumes converted to 0° C. and a pressure of 1.013 bar).
Prior efforts to control mercury emissions, in addition to other processes, have employed the addition of sulfide reagents to flue gas desulphurization (FGD) scrubbers. These sulfides have been alkali metal sulfides. The alkali metal are listed in group 1 of the periodic table and includes lithium, sodium, potassium, rubidium, cesium and francium. The alkali metal sulfide compounds previously employed for removing mercury were typically sodium sulfide, Na2S, potassium sulfide, K2S, and sodium tetrasulfide, Na2S4, as well as the alkali metal polysulfides, MSn. See, e.g., U.S. Pat. No. 6,214,304, and Babcock Power Environmental Inc. Technical Publication, “Multi-Pollutant Emissions Control & Strategies, Coal-Fired Power Plant Mercury Control by Injecting Sodium Tetrasulfide”; Licata A, Beittel R, Ake T, ICAC Forum, Nashville, Tenn. Oct. 14-15, 2003 and US Patent Application 2006/0094920. However, alkali metal/alkaline-earth metal sulfide blends and alkaline-earth metal dispersions have also been used. Some approaches utilize adsorbents such as powdered activated carbon, silicates, zeolites, clays and ash as solid supports for binding mercury on the surfaces of the solid. For example, U.S. Pat. No. 4,474,896 discloses an absorbent for mercury that is a polysulfide-containing adsorbent composition of a zeolite that has been treated to contain a metal cation capable of forming insoluble polysulfides when exposed to sulfanes in either vapor form or in an organic solution. In these absorbents the mercury vapor is first trapped in pores of a heterogeneous support that may incorporate sulfur containing compounds for reaction with mercury. However, each of these methods involve the use of a solid phase support, such as ion exchange resins or zeolites, and do not involve homogeneous phase reactions between mercury and the reactants in solution. These adsorbents, however, rely upon the surface area of the support to provide improved dispersion of the sulfur compounds. The preferred support materials have ion-exchange characteristics, and employ transition metals as the cation for the insoluble polysulfide. The surfaces available to adsorb the mercury eventually becomes saturated, and must be disposed of or regenerated. The method employed with these adsorbents involve passing the gas or liquid in contact with the support to allow adsorption of the mercury. U.S. Pat. No. 6,719,828 discloses a similar adsorbent using a solid substrate to support a polyvalent metal sulfide, and using sulfides and transition metals to bind to the mercury on the adsorbent surfaces. The adsorbent is injected into the flue gas to undergo a heterogeneous interaction with e mercury vapor.
Another form of adsorbent is disclosed in U.S. Patent Application 2007/0092418, which utilizes micro-porous sorbent particulates obtained from metal sulfates and metal sulfites by a high temperature reduction process that forms metal sulfides. These are solid phase particulates that act as a heterogeneous support substrate for the interaction between the mercury in the flue gas and sulfides in the particulates, where the mercury diffuses from the bulk flue gas to the solid surface of the particulates to react with the sulfides. U.S. Pat. No. 7,081,434 discloses impregnating fly-ash with sorbent materials to remove mercury from flue gas through a heterogeneous interaction. Many of these approaches, however, have not been very successful.
Mercury ions, Hg2+ react with sulfur, S2− to form sulfide compounds, HgS. This compound precipitates from solution as either metacinnabar, a black compound, or cinnabar, a red compound. Both forms are insoluble, however metacinnabar is unstable with respects to cinnabar, and will change into cinnabar slowly over time.
The precipitated mercury sulfide, in its black or beta-crystalline form, β-HgS, can be readily oxidized by pH or redox-induced reactions if is not stabilized, thereby rendering the mercury sulfide precipitate vulnerable to re-oxidation and resolubilization in water. This reoxidation of the reaction residues is greatly enhanced if the process contains alkali metal salts, such as sodium sulfate, sodium sulfite or sodium chloride. The presence of alkali metal ions renders the resultant black heavy metal sulfide more amorphous and less stable to re-oxidation and or resolubilization than a heavy metal sulfide prepared solely in the presence of alkaline-earth ions such as calcium or magnesium. This resolublization has the potential for increased bioavailability.
The dissolved mercury ions can react with trace amounts of elemental iron and iron II oxides carried with the dust of the combustion gases. The Hgx+ present becomes reduced in part to metallic mercury, Hg0, which being poorly soluble in water and highly volatile at moderately high temperatures is discharged as Hg0 vapor with the outgoing flue gases. This process wherein the outbound Hg0 concentration from the FGD scrubber exceeds its inbound concentration is termed ‘mercury re-emission’.
To overcome the above process product stability issues, a process was devised to incorporate solely alkaline-earth based components. See U.S. Pat Application 2005/0244319 and PCT Int. Appl. Publication WO/2008/008475. This process has a major drawback. Due to the relative insolubility and consequential slow reactivity of the alkaline-earth based components used it is necessary to carry out costly preparation of the reagent by fine milling the reagent mixture to an average particle size of approximately 3 micron prior to addition to the scrubber system. This is done such that its surface area of its active components can react with the mercury entering the scrubber in the few seconds of gas residence time available. This reagent system has limited utility owing to the tendency of the alkali earth based reagents to recrystallize and agglomerate with the result that particle size can double within a few days leading loss of chemical efficacy and handling problems due to settling of the reagent in a dense cake beneath clear supernatant liquor. Furthermore, prolonged contact of the sulfide and phosphate components leads to a loss of sulfide content attributed to accelerated oxidation catalyzed by phosphate. This oxidation can be as much as 20% within 2 days of preparation for a 20% w/w aqueous slurry of a blend of calcium carbonate, calcium sulfide, trisuperphosphate (in the ratios 4:4:1 or 3:2:1) and settling can be as much as 60% within a few hours of its preparation. Additionally, technical calcium sulfide is extremely abrasive in nature leading to excessive wear of grinding media as well as significant wear and tear to the check values of pumps used to mobilize calcium sulfide containing slurries. To overcome the difficulty of handling such materials expensive grinding media and costly equipment are required, e.g., 316 grade stainless steel pumps fitted with durable but expensive ceramic check-valves.
On entering a wet FGD system the ionized and oxidized portion of the total mercury (HgX) becomes largely dissolved in the scrubber SOx absorption media with approximately 5% of the inbound HgX passing through the system. Wet FGD absorption media is typically a dispersions of calcium carbonate, magnesium carbonate, or a mixture thereof and their respective sulphites and sulphates. It would be desirable to further decrease the amount of inbound HgX that simply passes through the system. Furthermore, it would be desirable to provide more efficient chemicals and systems for removal of mercury from combustion gases in general. The present invention now satisfies these desired and needs.